1. Field of the Invention
The present invention relates to a positive electrode active material containing a lithium-containing nickel oxide and to a lithium ion secondary battery equipped with the positive electrode active material.
2. Description of the Related Art
In recent years, electronic appliances such as portable personal computers and portable telephones are being miniaturized and made lightweight, and miniaturization and reduction of weight are being required for the secondary batteries used as power sources for these electronic appliances.
A lithium ion secondary battery using as a material of the negative electrode a substance capable of absorbing-desorbing lithium ions such as a carbon material has been developed as a secondary battery meeting the requirement described above, and has been put to a practical use as a power source for small electronic appliances. The secondary battery is smaller and lighter, and has a higher energy density, than a conventional lead accumulator or nickel-cadmium battery, and the demands for this particular secondary battery is on the increase.
A lithium-containing cobalt oxide (LiCoO2), which permits a high discharge potential and a high energy density to be obtained, has been put to a practical use as a positive electrode active material of the lithium ion secondary battery. However, the amount of natural resources of cobalt used as the raw material of the complex compound is very small, and the ore deposit that can be commercially utilized is unevenly distributed in a small number of countries. As a result, cobalt is costly and invites a large fluctuation in price. It follows that the cobalt supply in the future will be unstable.
Under the circumstances, researches into positive electrode active materials other than lithium-containing cobalt oxide have been vigorously carried out in recent years. For example, various compounds are reported in respect of the composite oxides between lithium and manganese, which are synthesized by various manganese raw materials and lithium raw material. To be more specific, lithium manganese composite oxide represented by LiMn2O4 having a Spinel type crystal structure is allowed to exhibit 3 V of potential relative to lithium by electrochemical oxidation and has a theoretical charge-discharge capacity of 148 mAh/g.
However, the lithium ion secondary battery using a manganese oxide or a lithium manganese composite oxide as the positive electrode active material gives rise to the defect that, when the secondary battery is used under an environment not lower than room temperature, the deterioration in the capacity of the secondary battery is markedly increased. It should be noted that the manganese oxide or the lithium manganese composite oxide is rendered unstable under high temperatures so as to cause Mn to elute into the nonaqueous electrolyte, giving rise to the defect noted above. Particularly, a large lithium ion secondary battery has been developed in recent years in various technical fields for use in an electric automobile or in road leveling. In the large lithium ion secondary battery, the heat generation during the use of the secondary battery is rendered non-negligible with enlargement in the size of the lithium ion secondary battery, with the result that the temperature inside the battery tends to be rendered relatively high even if the ambient temperature is close to room temperature. Also, even when it come to a relatively small battery used in, for example, a small portable electronic appliance, it is possible for the battery to be used under a high temperature environment such as within a room of a vehicle in the midsummer, with the result that it is possible for the temperature inside the battery to be rendered relatively high. Under the circumstances, it is very difficult to put a positive electrode active material using manganese as a raw material to practical use.
Researches on the nickel composite oxides are being vigorously carried out as post cobalt composite oxides. A nickel composite oxide, e.g., LiNiO2, exhibits a theoretical capacity of 180 to 200 mAh/g, which is larger than that of each of the LiCoO2 series active material and the LiMn2O4 series active material. In addition, LiNiO2 has an optimum discharge potential of about 3.6 V on the average and, thus, provides a highly hopeful positive electrode active material. However, the crystal structure of LiNiO2 is unstable, giving rise to the problem that the initial discharge capacity is greatly lowered in the charge-discharge cycle test with increase in the number of the charge-discharge cycles, and to the additional problem in safety that rupture and ignition are brought about in the nail sticking test of the lithium ion secondary battery prepared by using LiNiO2.
On the other hand, claim 1 of Japanese Patent Disclosure (Kokai) No. 10-326621 recites a secondary battery comprising a positive electrode using a lithium-containing metal composite oxide as an active material, a negative electrode using a metal composite oxide having an amorphous structure, and a nonaqueous electrolyte. Used as the active material of the positive electrode is a nickel-containing lithium composite oxide represented by LixNi1-yMyO2-zXa, where M represents at least one kind of an element selected from the group consisting of the elements of Group 2, Group 13, Group 14 of the Periodic Table and the transition metals, X represents a halogen atom, and x, y, z and a are defined to be 0.2<x≦1.2, 0≦y≦0.5, 0≦z≦1 and 0≦a≦2z.
Also, the heading [0010] of the Japanese Patent document quoted above refers to the desirable concentrations of impurities other than the elements Li, Ni, Co and M contained in the positive electrode active material represented by the structural formula given above. Specifically, the Japanese Patent document quoted above teaches that it is desirable for the concentration of the impurity Fe to be not higher than 0.01% by weight (not higher than 100 ppm), for the concentration of the impurity Cu to be not higher than 0.01% by weight (not higher than 100 ppm), and for the concentration of each of the impurities Ca, Na and sulfate group (SO4) to be not higher than 0.05% by weight (not higher than 500 ppm). Also, the Japanese Patent document quoted above teaches that it is desirable for the concentration of the water (H2O) to be not higher than 0.1% by weight.
What should be noted is that the elements other than those specified in the structural formula, i.e., Fe, Cu, Na and sulfate group SO4, are impurities in the Japanese Patent document in question and, thus, should desirably be contained in a smaller amount.